Is forced conversion of slaves allowed?

As explained in https://islam.stackexchange.com/questions/44673/is-idol-destruction-a-form-of-religious-intolerance The verse "Let there be no compulsion in religion" Applies only to the people from whom Jizyah is collected. So, my question is what is the ruling about non-muslim slaves? Are they also considered dhimma and their freedom of religion is protected? Or the ruling of freedom of religion does not apply to them?